ATM Anti-vibration foam, a suggestion

["Mark D. Holm" <mdholm@telerama.com>]

This isn't a current thread, but I'll speak up anyhow.  If I don't write while I'm
thinking of it, I may forget.

Frequently atm's are looking for vibration supressing foam for mounting motor driven
devices.  I can think only of one source that is locally available in the US and probably
Canada, perhaps in western Europe also: Ear plugs for noise supression.

Foam ear plugs designed to absorb noise are very good for their intended purpose, usually
achieving noise supression ratings higher than ear muffs.  In order to efficiently absorb
noise, the foam is made from polymers with a lossy mechanical characteristic.  That is,
when something causes the foam to move, much of the energy is absorbed and converted to
heat.  This is in contrast to more resilient foams in which motion causes the foam to
rebound.  You can see this effect easily with an ear plug.  Compress the foam a little
between your fingers, then release it suddenly.  The foam will rebound, but only very
slowly.  The slowness comes from the foam absorbing energy as it moves.

Sound absorbing ear plugs are sold in drug stores in my neighborhood.  I expect they are
sold throughout the US and probably Canada, possibly also western Europe.

There are two problems with the ear plugs for vibration isolation of motors on
telescopes.  1. They are a little small.  In most cases, one would probably have to cut
several slices of the same thickness and glue them to the mounting.  2. The foam isn't
quite as stiff as one would like for mounting motors that need to generate a reasonable
amount of torque, or transmit fine motions reliably.  To some extent, this problem can be
compensated by keeping the foam thin and using a large area of it.

Mark Holm
mdholm@telerama.com